Asymmetric compressor air extraction method

ABSTRACT

A method to mistune rotating stall aerodynamics by preventing the formation of coherent unsteady loads. Compressor air is extracted in an asymmetric circumferential pattern. The asymmetric bleed pattern disrupts the rotating stall cell rotational pattern, preventing the formation of a coherent aerodynamic excitation.

BACKGROUND OF THE INVENTION

Power generation industrial axial flow gas turbines are designed to optimally operate at a fixed rotational speed and output. In addition, traditional axial flow industrial gas turbine compressors have limited variable stage geometry and air extractions. These three factors, fixed speed operation, limited variable stage geometry and limited extractions lead to significant off-design aerodynamic conditions during start-up and shutdown operations. Rotating stall occurs in axial flow compressors during these off-design operations.

Rotating stall manifests itself as local stall cells that rotate at about half the wheel speed. These cells provide coherent unsteady aerodynamic loads on both the rotor and stator blades. As the rotor changes speed, the stall cell count changes thereby setting up different nodal diameters. The vibratory response on the rotor and stator blades from the rotating stall aerodynamic loads may lead to increased sensitivity of normal blade damage and premature failures.

BRIEF DESCRIPTION OF THE INVENTION

The invention improves axial flow compressor rotor and stator blade durability by eliminating or reducing the coherent aerodynamic forces created by rotating stall. More specifically, the invention provides a method to mistune the rotating stall aerodynamics thereby preventing the formation of coherent unsteady loads.

The invention may be embodied in a method of controlling air flow in a compressor comprising: flow disturbances created by part speed or off-design operation; extracting flow at a series of circumferentially asymmetric spaced positions at chosen axial portions of the compressor, thereby creating an extraction pattern to act against flow disturbances.

The invention may also be embodied in a method of controlling air flow in a compressor comprising: actuating extraction of compressor air, asymmetrically about a circumference of said compressor casing to act against a disturbance such as rotating stall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of rotating stall.

FIG. 2 is a schematic representation of asymmetric compressor air extraction as an example embodiment of the invention; and

FIG. 3 is a schematic perspective view of a compressor adapted for asymmetric compressor air extraction in accordance with an example embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Power generation industrial axial flow gas turbines are designed to optimally operate at a fixed rotational speed and output. In addition, traditional axial flow industrial gas turbine compressors have limited variable stage geometry and air extractions. These three factors lead to significant off-design aerodynamic conditions during start-up and shutdown operations. Rotating stall can occur in axial flow compressors during these off-design operations.

Rotating stall, schematically presented in FIG. 1, manifests itself as local stall cells 10 that rotate at about half the wheel speed (ω_(stall cells)≈½ ω_(engine)). These stall cells provide coherent unsteady aerodynamic loads on both the rotor and stator blades 12. As the rotor changes speed, the stall cell count changes thereby setting up different excitation characteristics known as nodal diameters. The vibratory response on the rotor and stator blades from the rotating stall aerodynamic loads may lead to increased sensitivity to normal blade damage and premature failures.

Improved axial flow compressor blade durability results from eliminating or reducing the coherent aerodynamic forces created by rotating stall. Reduction in these aerodynamic vibratory loads increases blade damage tolerance to normal operation damage such as tip rubs, corrosion and leading edge foreign object damage.

This invention improves axial flow compressor rotor and stator blade durability by eliminating or reducing the aerodynamic excitation on axial flow compressor rotor and stator blades from rotating stall. More specifically, the invention provides a method to mistune the rotating stall aerodynamics by preventing the formation of coherent unsteady loads.

Thus and more specifically, a novel manner of eliminating or reducing disturbances such as rotating stall in compressors is proposed, e.g., at a series of circumferentially spaced positions at one or more a chosen axial positions (stage) compressor air is selectively bled asymmetrically, in a circumferentially selective manner dependent upon the origin of the variations, to act against the rotating stall flow disturbances. In such a case, the air extraction process would be initiated, that is to say a rotating stall condition or potential stall condition would be countered by asymmetric extraction of air. No sensors are required as this is an operationally identified phenomenon—part speed and/or part load.

In an example embodiment of the invention, as schematically illustrated in FIGS. 2 and 3, compressor air is extracted through a series of generally shaped holes or slots 16 on the outer diameter flow path wall 18 in an asymmetric circumferential pattern. The extraction pattern is determined as a function of the rotating stall nodal diameter pattern as well as the aerodynamic strength of the cells. The asymmetric multi-stage bleed pattern disrupts the rotating stall cell rotational pattern, preventing the formation of a coherent aerodynamic excitation.

FIG. 3 schematically illustrates an axial flow compressor equipped with generally shaped air extraction holes or slots in accordance with an example embodiment of the invention. Generally shaped extraction holes or slots 16 are defined to selectively bleed compressor air through the compressor casing. The inlets are corrected by conduits (not shown) through respective fast acting shut-off valves (not shown) for removing the extracted air and re-routing it as deemed necessary or desirable. If similar air extraction holes are provided at a number of axial stations, the circumferentially corresponding extraction conduits at two or more stations may share the same control valve. If a circumferential series of extraction holes or slots are to be simultaneously actuated in ordinary course a circumferential series of extraction conduits may be connected to a common control valve. To economize on the extraction of high pressure air, the control unit should close the valve(s) once the stall cell has been suppressed. In one example embodiment, the extraction is continued for a predetermined period and then terminated. In another example embodiment, the extraction is gradually reduced after it is initiated. Other extraction protocols may be adopted as deemed necessary or desirable.

As mentioned above, to prevent the formation of coherent unsteady loads and thereby mistune the rotating stall aerodynamics, the invention provides an assembly wherein compressor air is extracted through a series of generally shaped holes or slots 16 on the outer diameter flow path wall 18 in an asymmetric circumferential pattern as the illustrated by way of example in FIG. 2. The extraction pattern may be experimentally or analytically determined as a function of the rotating stall nodal diameter pattern as well as the aerodynamic strength of the cells. These extractions may be located at a single, axial location or at several axial locations depending on the nature of the rotating stall, the required extraction flow and engine configuration restrictions. As can be seen in FIG. 3, this example embodiment presents an asymmetric extraction layout with two axial locations. The number of axial positions, circumferential arc lengths, extraction hole shape and number are defined based on the nature of the rotating stall. The asymmetric multi-stage bleed pattern disrupts the rotating stall cell rotational pattern, preventing the formation of a coherent aerodynamic excitation as conceptually illustrated in FIG. 2.

As is understood from the foregoing, the asymmetric bleed features include, at each specified axial location 20 on the compressor case 18, asymmetrically spaced extractions 16, within a specified arc length θ_(arc) at specified circumferential locations with defined extraction shapes, to provide the required total extraction flow. In an example embodiment, the arc length θ_(arc) is about 90 degrees.

For a split case application, the extractions 16 are placed on the top half case, thereby providing for easier field retrofit as well as improved accessibility for extraction manifolds and piping (not illustrated). As understood from the foregoing, the multi-stage extractions would require an extraction manifold to route the flow away from the unit.

Thus, the invention can provide the following benefits: 1) reduce/eliminate coherent unsteady aerodynamic excitation on blades; 2) retrofitable to existing engines with split case designs; 3) enabler for advanced aerodynamic blades; 4) improves compressor blade durability by increasing damage tolerance to tip rubs, corrosion pits and leading edge damage.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A method of controlling air flow in a compressor comprising: extracting high pressure air from a vicinity of or downstream of the chosen axial portion through a casing of the compressor, in a circumferentially asymmetric pattern to act against flow disturbances such as rotating stall.
 2. A method as in claim 1, wherein said extraction of high pressure air is through a plurality of holes or slots disposed asymmetrically about the circumference of the casing of the compressor.
 3. A method as in claim 1, wherein said casing is a split casing, and wherein said extraction of high pressure air is through a plurality of holes or slots disposed in a top casing part of said split casing.
 4. A method as in claim 1, comprising extracting high pressure air through a plurality of holes or slots disposed in an arc defined by the required flow volume and nature of the rotating stall formation.
 5. A method as in claim 4, wherein said holes or slots are asymmetrically disposed in said arc.
 6. A method as in claim 1, wherein said extraction is terminated after a predetermined period.
 7. A method as in claim 1, wherein said extraction may be gradually reduced after it is initiated.
 8. A method of controlling air flow in a compressor comprising: actuating extraction of compressor air, asymmetrically about a circumference of said compressor casing to act against a disturbance such as rotating stall.
 9. A method as in claim 8, wherein said extraction of high pressure air is through a plurality of holes or slots disposed asymmetrically about the circumference of the casing of the compressor.
 10. A method as in claim 8, wherein said casing is a split casing, and wherein said extraction of high pressure air is through a plurality of holes or slots disposed in a top casing part of said split casing.
 11. A method as in claim 8, comprising extracting high pressure air through a plurality of holes or slots disposed in an arc defined by the required flow volume and nature of the rotating stall formation.
 12. A method as in claim 11, wherein said holes or slots are asymmetrically disposed in said arc.
 13. A method as in claim 8, wherein said extraction is terminated after a predetermined period.
 14. A method as in claim 8, wherein said extraction may be gradually reduced after it is initiated. 